This invention relates to the surface protection of ceramic bodies, and, more particularly, to the surface protection of porous ceramic bodies such as those used as thermal protection tiles.
Ceramic bodies in the form of tiles are widely used to insulate underlying structures against the high temperatures encountered in some applications. As an example, an aerodynamic vehicle such as the space shuttle or a supersonic aircraft is heated by friction to a high temperature as it flies through the air. The temperature may be greater than the maximum permissible temperature of the metallic airframe and skin structure.
To insulate the airframe structure from the high temperature resulting from frictional heating, a portion of the outside of the skin of the aircraft may be covered by ceramic insulation that can withstand the temperatures experienced during flight. The ceramic insulation is normally provided as tiles of a ceramic material that are affixed to the exterior of the skin, which in turn is supported on the airframe. During flight, the outer surface of the ceramic insulation tile is heated to an extremely high temperature, but the interior surface of the tile remains relatively much cooler because of the poor thermal conductivity of the ceramic material. Consequently, the exterior surface of the skin also remains relatively much cooler than the outer surface of the insulation. The type and thickness of the ceramic insulation are selected to maintain the outer surface of the skin below its maximum permitted temperature.
One known type of ceramic insulation is a porous ceramic made by pressing together fibers of one of more ceramic materials. This fibrous, porous ceramic is resistant to damage from thermal shock and thermal cycling. However, it is relatively soft and can be damaged by external impact and wear forces. To lessen such damage, it is known to apply protecting coatings to the exterior surface of the ceramic insulation.
Several such protective approaches are discussed in U.S. Pat. No. 5,079,082. As described therein, an external protective coating made of a glass or glass/ceramic is applied to the surface of the ceramic insulation and fired. According to the '082 patent, in a prior approach it was known to apply the protective coating solely to the surface of the ceramic article, leaving a distinct interface between the coating and the underlying ceramic insulation. The result is that the protective coating may separate from the underlying ceramic as a result of mechanical or thermal shocks or other forces, a process termed spallation, leaving the underlying ceramic insulation unprotected and subject to accelerated damage. The '082 patent provides an improved protective layer having a diffuse, graded interface, which is intended to minimize the likelihood of such a separation by distributing the separation forces over a larger region than in the prior approach that produces a distinct interface.
While operable to some degree, the prior approaches have drawbacks when used in service applications. There is an interface between the protective layer and the ceramic insulation, albeit diffuse in some cases, that can be the source of spallation failure. Moreover, application of the protective coating requires that the ceramic insulation tile be fired at a high temperature. Thus, to repair a tile that has experienced some damage during service, the tile must be removed from the airframe and processed. This repair approach is expensive and difficult to accomplish when the aircraft is at a remote location unless stocks of all requisite tile configurations are maintained.
There is a need for an improved approach to protecting porous ceramic bodies from surface damage. The present invention fulfills this need, and further provides related advantages.